In recent years, an increasing interest has been devoted to energy storage technology. As the energy storage technology is applied to mobile phones, notebooks, personal computers, etc., and further extends its application to electric vehicles, research and development on energy storage is becoming more and more specialized. In view of this, electrochemical devices attract public attention, and among others, a major attention is focused on chargeable/dischargeable secondary batteries.
Among secondary batteries that are now in use, a lithium ion secondary battery developed in the early 1990s is in the spotlight because of its advantage of providing a higher drive voltage and a far greater energy density than those of conventional batteries, such as Ni-MH, Ni—Cd and sulfuric acid-lead batteries.
In general, such a lithium ion secondary battery includes cathode and anode capable of lithium ion insertion/emission, and an electrolyte transferring lithium ions between the cathode and the anode. With regard to this, a non-aqueous electrolyte solution containing an electrolyte salt dissolved in an organic solvent is used as the electrolyte.
However, the organic solvent contained in the electrolyte is highly volatile and flammable, and thus may cause a safety problem under overcharge, overdischarge, and high temperature conditions when applied to a lithium ion secondary battery.
To solve this problem, Japanese Patent Publication No. 2002-110225 discloses an imidazolium-based ionic liquid and an ammonium-based ionic liquid for use in a lithium ion secondary battery. However, such ionic liquids are problematic in that they are reduced at a voltage higher than the redox potential of a lithium ion in an anode, or imidazolium and ammonium cations may be intercalated into an anode along with lithium ions. Further, as a result of actually using the imidazolium-based ionic liquid or the ammonium-based ionic liquid alone as a liquid electrolyte for a lithium secondary battery, the ionic liquid was not suitable to be practically applied to a secondary battery due to a significant drop in the capacity of the secondary battery during repeated charge/discharge cycles. And, the ionic liquid is expensive and complicated to produce and filter.
Therefore, various attempts are currently being made to ensure the safety of a secondary battery by improving existing electrodes or electrolyte components, or developing novel electrode active materials or electrolytes.